The present invention relates to a method for forming a fine resist pattern in a process for fabricating a semiconductor integrated circuit (IC) device and the like (in this specification, such a method will be simply referred to as a "pattern forming method"), and also relates to a material for forming such a pattern used in the method (in a similar manner, such a material will be simply referred to as a "pattern forming material" in this specification).
Conventionally, in fabricating ICs, large-scale integrated circuits (LSIs) and the like, a pattern has been formed by a photolithography process using UV light. However, a light source utilizing a shorter wavelength has been used more and more frequently as the size of a semiconductor device has become increasingly small. Recently, in the case of using a light source utilizing a shorter wavelength, a surface imaging process using a dry development technique has been developed in order to increase the focal depth and to improve a practical resolution.
The surface imaging process is disclosed, for example, in U.S. Pat. No. 5,278,029. The patent discloses a negative type surface modification process. Specifically, according to the disclosed process, first, a polysiloxane film is selectively formed on the surface of a resist film, which is made of a resist material generating an acid when the material is exposed to light. Thereafter, the resist film is dry-etched by using the polysiloxane film as a mask, thereby forming a resist pattern.
Hereinafter, a conventional method for forming the resist pattern will be described with reference to FIGS. 9(a) through 9(d).
In the exemplary method to be described below, a copolymer of 1,2,3,4-tetrahydronaphthylideneimino-p-styrene sulfonate (NISS) and methyl methacrylate (MMA) is assumed to be used as a resist material generating an acid when the material is exposed to light.
First, as shown in FIG. 9(a), when a resist film 11, which is applied on a semiconductor substrate 10 and is made of a material generating an acid through the exposure to light, is irradiated with ArF excimer laser beam 14 by using a mask 13, an acid is generated in an exposed area 11a of the resist film 11. The generated acid contributes to turning the exposed area 11a into a hydrophilic area. As a result, water in the air can be easily adsorbed into the exposed area 11a. Consequently, a thin water-adsorbing layer 15 is formed in the vicinity of the surface of the exposed area 11a as shown in FIG. 9(b).
Next, when an alkoxysilane gas 16 is introduced onto the surface of the resist film 11, the acid, which has been generated on the surface of the exposed area 11a, works as a catalyst, thereby hydrolyzing and dehydrating alkoxysilane. As a result, a metal oxide film 17 is formed on the surface of the exposed area 11a, as shown in FIG. 9(c). Subsequently, when the resist film 11 is dry-etched in accordance with a reactive ion etching (RIE) technique using O.sub.2 plasma 18 while using the metal oxide film 17 as a mask, a fine resist pattern 19 is formed as shown in FIG. 9(d).
In this pattern forming method, a resist pattern is formed by performing the steps of: generating an acid in an exposed area of a resist film; selectively forming a metal oxide film in the exposed area by using the generated acid as a catalyst; and dry-etching the resist film by using the metal oxide film as a mask. Thus, this method is a negative type lithography process in which a resist pattern is formed in the exposed area of a resist film.
The negative type lithography process has the following problems in, for example, forming contact holes for connecting multi-layer interconnections of an integrated circuit.
First, the usage itself of a mask, generally employed in an exposure process step, causes the following problems. In the lithography process for forming contact holes, the opening ratio of the mask becomes very high if the negative type lithography process is used as described above. Specifically, a light blocking film against the exposing radiation is formed only in the portions corresponding to the contact holes on the mask. On the other hand, the light blocking film is removed and quartz of the mask substrate is exposed in the portions other than the contact hole portions, because the former portions can transmit the exposing radiation. In general, the ratio of the area occupied by all of the contact holes to the entire area of a semiconductor chip is very small. Thus, the opening ratio of the mask, i.e., the ratio of the area occupied by the exposed quartz to the area of the light blocking film on the mask becomes very high.
When the opening ratio of the mask becomes high, the process is much more likely to be affected by the contamination with ambient dust. More specifically, even when dust is adhered to the light blocking film portions of the mask, the dust hardly affects the process. However, if the dust is adhered to the transmissible portions of the mask, then these portions are turned into light blocking portions. When the exposure is performed by using such a mask to which dust has been adhered, pattern defects are caused in the portions to which the dust has been adhered and the portions corresponding thereto. As can be understood from the foregoing description, since the opening ratio of the mask becomes high in the negative type lithography process, the process is more likely to be affected by dust. As a result, this process has a problem in that the resulting yield is likely to decrease.
Next, a second problem will be described. In the lithography process for forming contact holes, a half-tone type mask is sometimes used in order to attempt to increase the focal depth. However, in most cases, the effect of increasing the focal depth can be attained only in a positive type lithography process and cannot be attained in the negative type lithography process. Thus, in forming contact holes, the focal depth becomes adversely small in the negative type process as compared with the positive type process.
The first and second problems described above are caused not only when the contact holes are formed, but also when a mask having a large light transmissible area is used and when the increase in focal depth is attempted.